Security systems have steadily increased in complexity over the years, beginning with the simple lock to the modern electronic security systems. Current security systems are not only designed to protect a home or commercial property from unauthorized intrusion but also to provide status of environmental conditions, such as temperature, air quality, fire warnings, carbon monoxide warnings, etc. Such systems include a myriad collection of sensors ranging from video cameras, infrared sensors, motion detectors, pressure sensors, temperature sensors, smoke detectors, and various air quality sensors. These sensors are distributed throughout a property and usually linked to a centralized security monitoring system.
To properly monitor an area, detectors must be appropriately positioned. During installation of a security system, the detectors must be installed at points where they have clear lines of sight so that the detector can efficiently monitor a maximum area. In situations where the area is irregularly shaped or contains objects that can obstruct a detector's field of view, multiple detectors are installed such that their fields of view partially overlap. In this way an area can be effectively monitored and the number of blind spots, i.e., regions in the monitored area that are not within the field of view of any of the detectors, is greatly reduced.
An additional consideration during installation of the detectors is aesthetics. With respect to aesthetics, most people would prefer not to have a plurality of detectors scattered throughout a room in a manner that would detract from the overall appearance of the area. In this regard, manufacturers have designed detectors to be visually appealing or compact so that they are less noticeable. Security system installers also position detectors in areas that draw minimal attention, such as corners or on ceilings or near the floor.
A further consideration is concealment. A detector that is visible to a would-be intruder is easier to defeat than one that is not visible. Often, detectors are installed behind vents or under furniture, thus limiting an intruder's chances of noticing the detector and employing a countermeasure. Consequently, a concealed detector would have a high chance of successfully detecting an intrusion.
However, both the aesthetic and concealment considerations can pose a serious problem during installation and even after installation. The problem in question is referred to as detector masking. Masking occurs when a detector is prevented from operating properly. Detector masking may be caused by any number of reasons, ranging from improper placement of the detector, accidental block of the sensor by an obstructive object, or even an intentional action in an attempt to thwart the detector.
One of the commonly used detectors is the passive infrared (PIR) motion detector. PIR motion detectors are electronic devices used in some security alarm systems to detect motion of an infrared emitting source, usually a human body.
All objects having a temperature above absolute zero (−273.15° C. or −459.67° F.) emit radiation according to the black body radiation model. Much of this radiation is invisible to the human eye, such as infrared radiation, but these invisible wavelengths can be detected by electronic devices designed for such a purpose. In the case of the PIR motion detectors, the wavelengths being detected fall into the infrared band. The PIR does not emit energy of any type but merely passively accepts infrared energy through an opening in its housing. The opening is usually covered with an infrared-transparent (but translucent to visible light) plastic sheet, which may or may not have Fresnel lenses molded into it. This plastic sheet prevents the intrusion of dust and insects while the Fresnel lenses, if present, focus the infrared energy onto the surface of an infrared sensor.
An intruder entering the monitored area is detected when the infrared energy emitted by the intruder is focused onto a section of the infrared sensor, which had previously been viewing at a much cooler part of the monitored area. That portion of the infrared sensor becomes warmer than when the intruder wasn't there. As the intruder moves, so does the hot spot on the surface of the infrared sensor. This moving hot spot causes the electronics connected to the infrared sensor to activate the detection input on the alarm control panel. Conversely, if an intruder were to try to defeat a PIR perhaps by holding some sort of thermal shield between himself and the PIR, a corresponding ‘cold’ spot moving across the face of the chip will also cause the relay to de-energize—unless the thermal shield has the same temperature as the objects behind it.
Unintentional masking may occur in situations where a piece of furniture or other such obstructive object is placed in front of a PIR motion detector. The PIR motion detector, being so obstructed, is unable to detect any motion. Indeed, since the obstructive object is most likely not to move, the PIR motion detector would not provide any indication of a problem. The PIR motion detector would simply register as no motion being detected.
Additionally, masking may occur due to environmental conditions unrelated to an intrusion. For instance, detectors for sensing temperature differences may be masked if direct sunlight or airflow from a ventilation system impacts the sensor. In such a case, the sensors would provide a false reading and thus not detect an actual temperature change for the coverage area. Thus, PIR motion detectors should not be placed in a location where direct sunlight may impact the infrared sensor, as this would artificially raise the detected temperature across the entire sensor surface such that an intruder's body temperature would be obscured.
A further masking event can be the result of an intruder attempting to defeat the PIR motion detector. While this masking is obviously the most serious, it is highly important to identify all masking situations. In the case of an intrusion, an alarm can be activated. Conversely, in the cases of an environmental condition-related or unintentional masking, the sensor can be repositioned or other action taken to correct the masking issue.
New security system standards include a requirement that detectors provide means for detecting a masking situation and alert a central monitoring unit when such masking occurs.